1. Field of the Invention
The present invention relates to an apparatus for measuring the viscosity and elasticity of a fluid such as a polymer melt. More particularly, the apparatus disclosed herein is particularly well suited for making such measurements in an "on-line" manner, during actual fluid processing. In accordance with the invention, such parameters are determined dynamically by measuring the characteristics of a continuously moving stream of fluid while such stream is being subjected to shear forces.
Changes in elasticity and viscosity are responsible for many of the variations in the processing and performance properties of polymer melts such as thermoplastics. In commercial use of such materials, the quality of the manufactured end product can usually be controlled successfully if such characteristics are known and controlled during the production process.
In the past, viscosity and elasticity of polymer melts were often measured "off-line". That is, a sample of the polymer melt was taken from the production apparatus, and then tested in a separate and often remote laboratory. However, because automated equipment can now make products from polymer melts at high speed, large numbers of products of poor quality, or products which do not conform to specifications, can be produced before the "off-line" results of viscosity and elasticity measurements are known.
Therefore, it is desirable to measure viscosity and elasticity of polymer melts, in an "on-line" procedure, during processing and production operations.
2. Description of the Prior Art
Various apparatus have been proposed which may be used to make measurements of the viscosity and elasticity of fluids such as polymer melts. Such apparatus include capillary or slit flow devices, and devices which measure rheological quantities by determining the shear characteristics of the polymer melt. The apparatus of the present invention is of the latter type, and operates dynamically on a continuous stream of fluid.
An example of shear measuring apparatus is the "Couette-Hatschek" rotating cylindrical viscometer, illustrated and described in Plastics & Polymers, February, 1973. That device includes concentric inner and outer cylinders. The inner cylinder is attached to a reduced diameter torsion cylinder which is, in turn, fixed to an upper ram mounted on the framework of the apparatus. A shaft is attached at one end to the inner cylinder and projects through the torsion cylinder out of the framework. A polymer melt is placed between the inner and outer cylinders and the outer cylinder is rotated to impart a shear force to the polymer material. The viscous drag of the material produces a torque, on the inner cylinder, which is interpreted as a shear stress.
The "Couette-Hatschek" viscometer, however, is prone to certain sources of error in measuring viscosity. One source of error is the requirement for an "end correction" to the cylinder length, which is necessitated by the viscous flow which takes place at locations other than in the annular gap between the cylinders. Complex secondary flows also occur at other locations in the fluid flow path and result in errors. Additionally, there are certain practical disadvantages to the "Couette-Hatschek" apparatus because the inner cylinder and torsion cylinder usually are made as two separate pieces.
A disclosure of viscometer measurements including a measurement of dynamic shear of a polymer melt, is set forth in a bulletin by H. K. Bruss RheoVerfahrenstechnik GmbH. However, the viscometer discussed therein incorporates an enlarged diameter inner cylinder or "bob" and a reduced diameter torsion measuring cylinder constructed from two separate components, thus giving rise to erroneous readings as described above.
U.S. Pat. No. 3,128,620 (Gupta) also relates to a torque tube rotational viscometer, for measuring the dynamic shear of a fluid. It includes a torque sleeve mounted concentrically within a cylindrical rotor and coupled to a torque transmitting shaft. The sleeve is mounted with an end cap assembly through a protective mantle assembly. The rotor encircles the sleeve only along a portion of the length of the sleeve and, therefore, fluid is conducted first to a circular region defined between an outer body and the sleeve, and thereafter to an annular region between the rotor and the sleeve. Thus, this device is characterized by certain of the same drawbacks found in other prior art apparatus discussed above.
A viscometer similar to the "Couette-Hatschek" rotating cylinder viscometer is also disclosed in U.S. Pat. No. 2,817,231 (Barstow).